About Me

Friday, 17 July 2015

The Seasons – Why summers are hot and winters are cold in the N-hemisphere, and CO2 levels oscillate over the course of a year?

“Seasons, more than anything else, influence human, animal and

vegetation activities, over the course of a year”

I
am often asked about the seasons and what causes them? I thought it might be a
good idea to have a separate blog entry for this interesting subject. What I found particularly exciting that one
can easily calculate the maximum angle that the Sun will reach during each
season – I shall calculate it for Glasgow, Scotland but the recipe does it for
any latitude. Seasons in the Southern
hemisphere are exact opposite to those in the Northern hemisphere but the
science is the same.

Our
Earth rotates round the Sun in an elliptical orbit, and its orbit describes a
plane - the ecliptic plane. The Earth is
spherical – a bit flattened at the poles due to the large centrifugal forces
during its cooling down a few billion years ago and it spins about an axis
passing through the North and South poles.
Seasons happen because the Earth’s spin axis is not perpendicular to the
ecliptic plane but is tilted at 23.5 degrees to the Earth’s plane of rotation
around the Sun. This means that during
the summer, the N-hemisphere is tilted towards the Sun and receives more light
at a steeper angle while in winters, after six months, the situation is
opposite to that in the summers. The days are longer during the summer while in
winters the days are shorter. At Equinox,
in March and September, the days are 12 hours long. This is best explained through pictures; the
slides follow below.

The lowest elevation of the Sun at winter solstice is (90 – latitude
– 23.5) degrees

The highest elevation of the Sun at summer solstice is (90 –
latitude + 23.5) degrees

Glasgow at 56 degrees has elevations of 10.5 and 57.5 degrees at winter and summer solstice times. That is why it always seems dark in Scotland during December!

London is at 51.5 degrees, Sun elevations will be 15 and 62
degrees at winter and summer solstices.

For New York (latitude = 40.7), they are 25.8 and 72.8
respectively.

Notice that the difference in Sun’s elevation at the solstices
is 47 degrees at all places

At tropic of Cancer (latitudes +23.5), the Sun will be at 90 degrees (straight overhead and very warm) at summer solstice.

At 66.5 degrees latitude the Sun elevation is zero at winter solstice
and it is below horizon on that day.

The Sun’s elevation determines the length of
shadows cast on the ground. Higher
elevation gives a shorter shadow (see slides below) and sunlight falls on a
smaller area making the Sun feel hotter during the summer. Reverse is true in the winter.

Additionally, at higher latitudes (see slide 3) sunlight travels through a greater length of the atmosphere than at lower latitudes. This causes more absorption of energy and the sunlight carries less energy.

Another interesting observation is the variation of
measured CO2 levels over the year.
In the N-hemishpere, CO2 levels are about 4% lower at the end
of the summer than at the start of spring.
This is simply because during the summer, the vegetation grows rapidly
and uses up lot of CO2 present in the atmosphere. During the winter, photosynthesis is much
lower and uptake of CO2 is corresponding reduced. Dead vegetation also returns some of the CO2
back to the atmosphere giving a peak in its levels. This is shown clearly in
the slides. Another interesting
observation is that the difference in CO2 levels during the summer
and winter in the southern hemisphere is much less pronounced than in the
northern hemisphere because much more of the northern hemisphere has forests
and green area than the southern.